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<h1><a name="SECTION0071300000000000000000">
5.13 <tt class="module">UserList</tt> --
         Class wrapper for list objects</a>
</h1>

<p>
<a name="module-UserList"></a>

<p>
<span class="note"><b class="label">Note:</b>
This module is available for backward compatibility only.  If
you are writing code that does not need to work with versions of
Python earlier than Python 2.2, please consider subclassing directly
from the built-in <tt class="class">list</tt> type.</span>

<p>
This module defines a class that acts as a wrapper around
list objects.  It is a useful base class for
your own list-like classes, which can inherit from
them and override existing methods or add new ones.  In this way one
can add new behaviors to lists.

<p>
The <tt class="module">UserList</tt> module defines the <tt class="class">UserList</tt> class:

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><span class="typelabel">class</span>&nbsp;<tt id='l2h-838' xml:id='l2h-838' class="class">UserList</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>list</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Class that simulates a list.  The instance's
contents are kept in a regular list, which is accessible via the
<tt class="member">data</tt> attribute of <tt class="class">UserList</tt> instances.  The instance's
contents are initially set to a copy of <var>list</var>, defaulting to the
empty list <code>[]</code>.  <var>list</var> can be either a regular Python list,
or an instance of <tt class="class">UserList</tt> (or a subclass).
</dl>

<p>
In addition to supporting the methods and operations of mutable
sequences (see section <a href="typesseq.html#typesseq">3.6</a>), <tt class="class">UserList</tt> instances
provide the following attribute:

<p>
<dl><dt><b><tt id='l2h-839' xml:id='l2h-839' class="member">data</tt></b></dt>
<dd>
A real Python list object used to store the contents of the
<tt class="class">UserList</tt> class.
</dl>

<p>
<strong>Subclassing requirements:</strong>
Subclasses of <tt class="class">UserList</tt> are expect to offer a constructor which
can be called with either no arguments or one argument.  List
operations which return a new sequence attempt to create an instance
of the actual implementation class.  To do so, it assumes that the
constructor can be called with a single parameter, which is a sequence
object used as a data source.

<p>
If a derived class does not wish to comply with this requirement, all
of the special methods supported by this class will need to be
overridden; please consult the sources for information about the
methods which need to be provided in that case.

<p>

<span class="versionnote">Changed in version 2.0:
Python versions 1.5.2 and 1.6 also required that the
                constructor be callable with no parameters, and offer
                a mutable <tt class="member">data</tt> attribute.  Earlier versions
                of Python did not attempt to create instances of the
                derived class.</span>

<p>

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